In UNIX, the system call to delete/remove file is "unlink" and you can see that the "path" of the file to be deleted is the only parameter passed to the function call and that's why we de-reference the pointer using copyinstr(arg0)

# man -s 2 unlink
System Calls unlink(2)
NAME
unlink, unlinkat - remove directory entry
SYNOPSIS
#include
int unlink(const char *path);
int unlinkat(int dirfd, const char *path, int flag);
DESCRIPTION
The unlink() function removes a link to a file. If path
names a symbolic link, unlink() removes the symbolic link
named by path and does not affect any file or directory
named by the contents of the symbolic link.
Otherwise, unlink() removes the link named by the pathname
pointed to by path and decrements the link count of the file
referenced by the link.

Wanna to learn that, here are some tutorial materal I gathered from the web: